A power electronic device such as a switching power supply, an inverter, a lighting circuit of a lighting device, or the like has a power converting circuit for converting power. The power converting circuit has a switching circuit for converting direct current to alternating current having rectangular waveforms. Consequently, the power converting circuit generates a ripple voltage having frequency equal to switching frequency of the switching circuit, and noise accompanying the switching operation of the switching circuit. The ripple voltage and noise exerts adverse influence on the other devices. Consequently, it is necessary to provide means for reducing the ripple voltage and noise between the power converting circuit and the other devices or a line.
As means for reducing the ripple voltage and noise, a filter including an inductance element (inductor) and a capacitor, so-called an LC filter is often used. The LC filters include a filter having one inductance element and one capacitor, a T filter, and a π filter. A common noise filter for preventing electromagnetic interference (EMI) is one of LC filters. A general EMI filter is configured by combining discrete elements such as a common-mode choke coil, a normal-mode choke coil, an X capacitor, a Y capacitor, and the like.
Recently, power-line communication is regarded as a promising communication technique used at the time of configuring a communication network in a house and is being developed. In the power-line communication, communication is performed by multiplexing a high frequency signal on a power line. In the power-line communication, noise occurs on a power line by various electric/electronic devices connected to the power line, and it causes deterioration in communication quality such as increase in the error rate. Consequently, means for reducing noise on the power line is necessary. In the power-line communication, it is necessary to prevent a communication signal on an indoor power line from leaking to an outdoor power line. The LC filter is also used as means for reducing noise on the power line and preventing a communication line on an indoor power line from leaking to an outdoor power line.
Noises propagating through two conductive lines include a normal-mode (differential-mode) noise generating a potential difference between two conductive lines and a common-mode noise propagating in the same phase in the two conductive lines.
Japanese Patent Laid-open No. Hei 9-102723 describes a line filter using a transformer. The line filter has a transformer and a filter circuit. A secondary winding of the transformer is inserted in one of the two conductive lines for transmitting power to be supplied from an alternate current power supply to a load. Two input terminals of the filter circuit are connected to both ends of an alternate current power supply, and two output terminals of the filter circuit are connected to both ends of a primary winding of the transformer. In the line filter, a noise component is extracted from a power supply voltage by the filter circuit and supplied to the primary winding of the transformer, and the noise component is subtracted from the power supply voltage on the conductive line in which the secondary winding of the transformer is inserted. The line filter reduces the noise in the normal mode.
Japanese Patent Laid-open No. Hei 5-121988 (FIG. 1) discloses a low-pass filter including three impedance elements. The low-pass filter has two high-impedance elements inserted in series in one of two conductive lines, and a low-impedance element whose one end is connected between the two high-impedance elements and whose other end is connected to the other one of the two conductive lines. Each of the two high-impedance elements includes a parallel connection circuit of a coil and a resistor, and the low-impedance element includes a capacitor. The low-pass filter reduces the normal-mode noise.
Japanese Patent No. 2,784,783 (FIG. 6) discloses a normal-mode noise filter circuit for reducing the normal-mode noise, and a common-mode noise filter circuit for reducing the common mode noise. The normal-mode noise filter circuit is configured to have two coils inserted in two conductive lines and a capacitor connecting some midpoints in windings of the coils. The common-mode noise filter circuit is configured to have two coils inserted in the two conductive lines and two capacitors provided between some midpoints in the windings of the coils and the ground.
Each of the conventional LC filters has a peculiar resonance frequency determined by inductance and capacitance, so that it has a problem such that a desired attenuation can be obtained only in a narrow frequency range.
The filter inserted in the conductive line for power transmission is requested to obtain a desired characteristic in a state where current for power transmission flows and to have a countermeasure against temperature rise. Consequently, the inductance element in the filter for the power converting circuit uses, as a magnetic core, a ferrite core with a gap. However, the characteristic of such an inductance element is similar to that of an inductance element using an air core, so that there is a problem such that, to realize a desired characteristic, the size of the inductance element becomes large.
In the line filter disclosed in Japanese Patent Laid-open No. Hei 9-102723, the coupling coefficient of the transformer is 1. If the filter circuit does not exert influence on the line filter, theoretically, the noise component can be completely removed. In reality, however, it is impossible to set the coupling coefficient of the transformer to 1, and the attenuation characteristic deteriorates as the coupling coefficient decreases. In particular, in the case where the filter circuit is configured by the capacitor, a series resonance circuit is configured by the capacitor and the primary winding of the transformer. Consequently, the impedance of a signal path including the capacitor and the primary winding of the transformer decreases only in a narrow frequency range close to the resonance frequency of the series resonance circuit. As a result, with the line filter, the noise component can be removed only in the narrow frequency range. From the above, a line filer actually configured has a problem such that it cannot efficiently remove a noise component in a wide frequency range.
Since the noise reduction principle of the low-pass filter described in Japanese Patent Laid-open No. Hei 5-121988 (FIG. 1) and the filter circuit described in Japanese Patent No. 2,784,783 (FIG. 6) is similar to that of the conventional LC filter, they also have a problem similar to that of the conventional LC filter.
In the respective countries, various controls are established over noise, that is, noise terminal voltage leaked from an electronic device to the outside via an AC power supply line. For example, CISPR (International Special Committee on Radio Interference) sets the control on noise terminal voltage in the frequency range of 150 kHz to 30 MHz. In the case of reducing noise in such a wide frequency range, particularly, on reduction of noise in a low frequency range of 1 MHz or less, the following problem occurs. Specifically, in the frequency range of 1 MHz or less, the absolute value of the impedance of a coil is expressed by 2πfL where the inductance of the coil is L and frequency is f. Therefore, generally, to reduce noise in the frequency range of 1 MHz or less, a filter including a coil having large inductance is necessary. As a result, the filter becomes large.